Reduce header box height on Platform Pro

Hi. Please can you tell me how to adjust the height of the header space on Platform Pro. I uploaded my own header but chose to keep it to a height of only 100 px so as not to waste real estate above the page fold. The box it plugs into though appears to be of fixed height so I am not achieving my aim. I am not yet competent in code etc. so please answer in simple language.
Regards
Neal Allen

Can you send a link to the site?
In one of my sites, this is the code I used. Perhaps this can orient you. You can add it to the CUSTOM CSS tab of the PLATFORM PRO settings or to your platformbase base.css - however you are handling customizations.
.branding_wrap {
height: 100px;
}

Hi I've tried to do that but I must have done the wrong thing. It doesn't work. The site is a new one that I have just started building and I am finding Page-lines to be brilliant but very difficult for a novice like me. Here is a link to the site,